In gravure printing, minute concave portions (cells) are formed on a gravure cylinder in accordance with plate-making information to produce a printing surface, and the cells are filled with ink so that the ink is transferred to an object to be printed. In a general gravure cylinder, a cylindrical iron core or aluminum core (hollow roll) is used as a base, a plurality of layers such as an underlying layer and a separation layer are formed on an outer peripheral surface of the base, and a copper plating layer (plating material) for forming a printing surface is formed on the plurality of layers. Then, cells are formed on the copper plating layer in accordance with plate-making information by a laser exposure apparatus, and thereafter, the resultant base is plated with chromium or the like for enhancing printing durability of a gravure cylinder. In this manner, plate making (production of a printing surface) is completed.
Conventionally, as a method and apparatus for performing copper plating on an outer peripheral surface of a gravure cylinder, the use of a phosphorus-containing copper ball as a soluble anode is well known. According to the conventional method and apparatus, both ends in a longitudinal direction of a gravure cylinder are held so as to be rotated and energized by a pair of roll chucks, the gravure cylinder is accommodated in a plating bath in which a plating solution is stored while the gravure cylinder is being rotated, and a current with a current density of about 10 to 15 A/dm2 is allowed to flow between the phosphorus-containing copper ball (soluble anode) in the plating solution and the gravure cylinder (cathode), to thereby deposit copper on an outer peripheral surface of the gravure cylinder, which functions as a cathode, with the result that copper plating is performed (for example, see Patent Documents 1 and 2).
However, in general, a phosphorus-containing copper ball used in a copper plating method and apparatus for a gravure cylinder contains 350 to 700 ppm of phosphorus and 2 to 5 ppm of oxygen, and the rest of the ball contains copper and impurities. Due to the impurities contained in the ball inevitably, anode sludge is generated during plating treatment, which causes defects such as rashes (minute protrusions) and pits (pinholes) on the outer peripheral surface of the gravure cylinder. Although there is a phosphorus-containing copper ball of high purity for producing a semiconductor and the like, such a ball is expensive and is not adopted for a gravure cylinder in terms of cost-efficiency. Further, in order to prevent the dissolution amount of a phosphorus-containing copper ball in a copper plating solution from increasing excessively to enhance the copper ion concentration, making it impossible to perform appropriate plating treatment, it is also necessary to dilute the solution by removing a plating solution periodically, thereby adjusting the copper ion concentration appropriately and disposing of a waste liquid. Further, a current is concentrated in the vicinity of both ends of the gravure cylinder, and hence the peripheral surface in the vicinity of both ends is plated thicker than a body portion, with the result that it is necessary to separately perform treatment for obtaining a uniform thickness of plating by follow-up polishing or the like.
On the other hand, in addition to a method using a phosphorus-containing copper ball as a soluble anode, a copper plating method using an insoluble anode is known. As a copper plating method and apparatus for a gravure cylinder using an insoluble anode, for example, a titanium plate coated on the surface with iridium oxide or the like is used as an insoluble anode, a plating bath and a copper dissolution bath are prepared, the copper plating material (e.g., copper oxides or copper carbonates) is dissolved in the dissolution bath, the resultant solution is supplied to a plating solution in the plating bath, and a current is supplied between an insoluble anode and a gravure cylinder forming a cathode. In this manner, copper plating is performed (for example, see Patent Document 3).
According to the above-mentioned method and apparatus, anode sludge is not generated so that defects such as rashes and pits are not caused, but there is still a problem that the peripheral surface in the vicinity of both ends of a gravure cylinder is plated thick. In order to solve this problem, the applicant of the present application has already proposed a copper plating method and apparatus for a gravure cylinder in which an insoluble anode positioned below a gravure cylinder is configured so as to be lifted in a plating bath, and the insoluble anode is brought close to a lower surface of the gravure cylinder with a gap of 5 mm to 30 mm in accordance with gravure cylinders of various sizes, with the result that a current is not concentrated in the vicinity of both ends of the gravure cylinder, plating with a uniform thickness can be performed over the full length of the gravure cylinder, and the concentration of copper and the concentration of sulfuric acid in the plating solution can be adjusted automatically (see Patent Document 4).
Still further, in the above-mentioned proposal, there are the following problems. That is, an insoluble anode is placed directly in the plating solution, and hence the consumption amount of additives such as a brightener and a burn prevention agent is remarkably large. A current density is about 15 to 20 A/dm2 and a voltage is about 10 to 15 V for the purpose of preventing a burn, and hence plating treatment takes a long time, which results in a large power supply cost. The uniformity of a plating thickness is insufficient. The insoluble anode is positioned below the gravure cylinder, and hence visibility and operability are poor. Considering these problems, the applicant of the present application has already proposed a copper plating method and apparatus for a gravure cylinder, in which a hollow cylindrical gravure cylinder is held at both ends in a longitudinal direction and accommodated in a plating bath filled with a copper plating solution, the gravure cylinder is rotated at a predetermined speed and supplied with a current so as to become a cathode, and a pair of anode chambers in the shape of a long box that is vertically installed slidably so as to face both sides of the gravure cylinder in the plating bath and contains insoluble anodes supplied with a current so as to become an anode are brought close to both side surfaces of the gravure cylinder with a predetermined interval to perform copper plating on an outer peripheral surface of the gravure cylinder (Patent Document 5).
According to the above-mentioned proposal, a copper plating method and apparatus for a gravure cylinder that provide good visibility and operability can be provided, in which copper plating with a uniform thickness compared to the conventional example can be performed over the full length of a gravure cylinder without generating defects such as rashes and pits irrespective of the size of the gravure cylinder, the concentration of a copper plating solution can be managed automatically, the consumption amount of additives can be reduced, plating treatment can be performed in a short period of time, and a power supply cost can be reduced. However, from the viewpoint of the uniformity of a thickness of copper plating over the full length of a gravure cylinder 300, the uniformity is not necessarily sufficient, and the following phenomenon has not been solved sufficiently. That is, in the vicinity of both ends of the gravure cylinder 300 (particularly, portions of about 50 mm to 200 mm from both ends), a current is concentrated, and hence, a peripheral surface in the vicinity of each end is plated thicker than a body portion, with a result that a thick plating layer of about 150 μm is formed.
The applicant of the present application has further continued to study extensively, and obtained a new landmark finding that, by dividing an insoluble electrode and adjusting a potential of each divided electrode, the current concentration in cylinder end portions can be prevented effectively. Thus, the applicant of the present application has provided a plating method for a cylinder and an apparatus therefor that provide good visibility and operability, in which copper plating with a more uniform thickness can be performed over the full length of a cylinder without generating defects such as rashes and pits irrespective of the size of the cylinder, the concentration of a copper plating solution can be managed automatically, the consumption amount of additives can be reduced, plating treatment can be performed in a short period of time, a power supply cost can be reduced, and the vicinity of both ends of the cylinder is capable of greatly preventing from being plated thicker than the body portion, to thereby eliminate or simplify treatment for obtaining the uniform thickness of plating, such as follow-up polishing (Patent Document 6).
The above-mentioned plating method for a cylinder is a plating method for a cylinder in which a long cylinder is held at both ends in a longitudinal direction and accommodated in a plating bath filled with a plating solution, the cylinder is rotated at a predetermined speed and supplied with a current so as to become a cathode, and a pair of electrode chambers in the shape of a long box that is vertically installed slidably so as to face both sides of the cylinder in the plating bath and contains insoluble electrodes supplied with a predetermined current are brought close to both side surfaces of the cylinder with a predetermined interval to perform plating on an outer peripheral surface of the cylinder. In this method, the insoluble electrode is divided into a large number of divided electrodes, and the insoluble electrode portions at least corresponding to the vicinity of both ends in a longitudinal direction of the cylinder are respectively divided into at least three divided electrode groups. Each divided electrode group has one or more divided electrodes, and a potential of the divided electrode group is controlled so as to adjust a thickness of a plating layer on the outer peripheral surface of each end of the cylinder (Patent Document 6, claim 1).
Further, the above-mentioned plating apparatus for a cylinder includes a plating bath to be filled with a plating solution, chuck means for holding a long cylinder at both ends in a longitudinal direction so as to be rotated and energized, and accommodating the cylinder in the plating bath, and a pair of electrode chambers in the shape of a long box that is vertically installed slidably so as to face both sides of the cylinder in the plating bath and contains insoluble electrodes supplied with a predetermined current, the electrode chamber being brought close to both side surfaces of the cylinder with a predetermined interval to perform plating on an outer peripheral surface of the cylinder. The insoluble electrode is divided into a large number of divided electrodes, and the insoluble electrode portions at least corresponding to the vicinity of both ends in a longitudinal direction of the cylinder are respectively divided into at least three divided electrode groups. Each divided electrode group has one or more divided electrodes, and a potential of the divided electrode group is controlled so as to adjust a thickness of a plating layer on the outer peripheral surface of each end of the cylinder (Patent Document 6, claim 10).